The present disclosure relates generally to thermal neutron logging tools and, more particularly, to matching the lithology response of thermal neutron logging tools having different source energies.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Thermal neutron logging devices have been used in the oil field for many years to measure formation porosity and other properties. These devices typically include a neutron source and a pair of thermal neutron detectors respectively located “near” and “far” relative to the neutron source. The neutron source may emit neutrons into a surrounding formation, which may scatter the neutrons and cause the neutrons to lose energy. Based on counts of these neutrons, thermal neutron logging devices may determine formation porosity and other properties because neutrons detected by a neutron detector placed not too close to the source may be largely dependent on the effect of elastic scattering on hydrogen nuclei in the formation. That is, the more hydrogen that is present in the formation, the fewer neutrons that arrive at such a neutron detector. Since formation porosity is generally water or hydrocarbon-filled, the count rate in the neutron detector may also be a measure of porosity. Because such devices may employ thermal neutron detectors, however, these devices may also be sensitive to the presence of thermal neutron absorbers in the subsurface environment. In particular, chlorine has a large capture cross section and saline fluids, of which chlorine is a component, are commonly encountered downhole. To reduce the sensitivity of such devices to this and other unwanted effects, the neutron porosity is typically derived from a ratio of count rates from the “near” neutron detector to the “far” neutron detector.
In addition to borehole fluids, many common downhole minerals may contain bound water or hydroxyls, so the hydrogen response of a thermal neutron logging device may not simply derive from porosity alone. Accordingly, while optimized for hydrogen sensitivity, thermal neutron logging devices may also have a residual sensitivity to other elements. This residual sensitivity to other elements may be referred to as the “lithology effect,” defined as the apparent porosity that the device computes minus the apparent porosity that the device would compute if placed in a standard formation (generally taken to be calcite) with the same true porosity. To make explicit these departures from a true porosity reading, the measurement of such a thermal neutron logging device has come to be called “thermal neutron porosity.” While these departures may appear to be a shortcoming of the measurement, over the years, a substantial body of experience has accumulated on the profitable use of these differences (e.g., as an indicator of lithology and in particular shale in the formation). These measurements may also be used in a comparison mode, where the correctness of the porosity reading may be less important than its consistency from well to well and over time.
Historically, thermal neutron porosity devices have typically employed an AmBe radioisotopic neutron source, which emits neutrons of a range of energies with an average value of around 4 MeV. For a variety of reasons, it would be very desirable to replace radioisotopic neutron sources with electronically-controlled neutron generators. Such neutron generators have been available for many years, many of which may emit 14 MeV neutrons based on deuterium-tritium (d-T) reactions. If a neutron generator is used in place of an AmBe neutron source in a thermal neutron porosity device, the differences in neutron source energy may substantially modify the response of the device to various downhole materials. For example, in addition to elastic neutron scattering, neutrons above 1 MeV may encounter a number of isotope-specific inelastic neutron reactions. Moreover, for a thermal neutron porosity device employing a 14 MeV neutron source, the contribution of these reactions to the total neutron response may be much larger than for a similar device employing a 4 MeV neutron source. If a thermal neutron device employing a 14 MeV neutron source were intended only to measure porosity, these differences could be accounted for in the device design, but since the neutron source energy of an electronic neutron generator differs substantially with that of AmBe source, substantial differences in the lithology response may also result.